The present invention relates to flame monitoring systems for vapor generators of the type having a furnace equipped with tilting burners. More particularly, the invention relates to a scanner system for detecting the presence of flame at each of the individually tilting burners in tangentially-fired boilers.
In the operation of a vapor generator there exists the danger of emitting fuel to the burners and thence into the furnace combustion chamber when there is no flame in which to provide ignition energy to ignite and burn the fuel. This condition results in a creation of a furnace atmosphere which is highly explosive. To ensure safe operation of the furnace, it is customary to provide some means of monitoring the furnace chamber to detect the presence of flame therein so that the supply of fuel to the furnace occurs only when the flame is present, thereby preventing the establishment of an explosive atmosphere within the furnace chamber.
One common method of firing fossil fuels, such as coal, oil or natural gas, in a furnace of a vapor generating boiler is known as tangential firing. In this method, fuel and combustion air are introduced into the furnace through burners, often termed fuel admission assemblies, located in the corners of the furnace alternatively stacked between air admission assemblies in a vertical array, termed windbox, of typically three or more burners per corner. The fuel and air streams discharging from the burners in the air admission assemblies respectively are aimed tangentially to an imaginary circle about the center of the furnace chamber. This creates a fire ball in the middle of the furnace chamber which serves as a continuous source of ignition for the incoming fuel. More specifically, a flame is established in one corner which in turn supplies the required ignition energy to stabilize the flame emanating from the corner downstream and laterally adjacent to it.
A distinct advantage of the tangential firing concept is that a wide range of control of steam temperature can be obtained by tilting in unison the nozzle tips of the burners and the air admission assemblies of the corner windboxes upward or downward. By so doing, the fireball is physically raised or lowered within the furnace so as to increase or decrease the heat absorption from the furnace water walls thereby effecting wide range control over the temperature of the combustion gases leaving the combustion zone and passing over downstream superheat and reheat surface. By tilting upward as load decreases, low load operation can be achieved while holding the overall cycle efficiency and maintaining better operation of the turbine. Additionally, the vertical adjustability of the burner and air admission assembly nozzle tips permits the operator of the furnace to compensate the changes in heat absorption within a furnace water wall resulting from fuel variation, and in particular, for the variations in the amount of slagging of the furnace water wall when coal is fired within the furnace.
In a furnace employing the tilting tangential firing system, it is desirable to monitor not only the fireball formed in the middle of the furnace chamber but also to monitor the flames of the individual corner burners to detect the existence of the ignition of the fuel emanating from each of the individual burners. A problem unique to furnaces equipped with tilting burners is that the flame emanating from the individual corner burners moves vertically as the burners are tilted upward or downward for steam temperature control. Thus, one must provide a flame scanner which is capable of viewing the individual flame emanating from a burner over the entire range of burner tilt while at the same time ensuring that the flame scanner has a view restricted as to view only the flame emanating from the burner with which it is associated and not the fireball or the flames of neighboring burners.
One common method of addressing the above-mentioned problem employs a plurality of flame scanners, one per burner, each mounted in the corner windboxes and aligned to sight through the flame emanating from its associated burner nozzle so as to view the region immediately in front of that burner. One example of such a flame monitoring system is discussed in detail in U.S. Pat. No. 3,241,595. The scanner sensor is mounted in the burner nozzle tip at the furnace end of the burner and is equipped with a flexible metallic sleeve through which wires from the scanner sensor bear back through the windbox to the scanner controls. The flexible metallic sleeve is provided to permit the scanner sensor to tilt with the nozzle tip in which it is mounted thereby allowing the scanner sensor to continuously view the flame emanating from the burner and only that burner. A problem associated with this arrangement is that the scanner sensor is exposed to direct radiation from the flame it views which may have a temperature in excess of 1400 C. Being exposed to such radiation would soon destroy the sensor unless the sensor is effectively cooled.
Another approach, as shown in U.S. Pat. No. 4,168,785, to solving the above-mentioned problem employs a plurality of flame scanners, one per burner, each positioned with a sensor viewing through a port in the furnace wall at a location adjacent its associated corner burner and aimed to sight transversely across the path of the flame emanating on that burner. Each scanner is pivotally mounted in a track outside the furnace so that the entire scanner assembly is tilted accordingly so as to permit the sensor to follow the flame emanating from its associated burner as the burner nozzle tips tilt upward or downward. Such a system has a distinct disadvantage having to provide a control system to ensure that each scanner follows its associated burner during the tilt maneuvers in order to prevent the scanner from losing sight of the flame and erroneously shutting down a properly-operating burner.